This exclusive Jack Skellington Funko Pop #28 shirt was a Hot Topic exclusive made for San Diego Comic Con in 2015. There's still a couple left floating around.
The #28 on the shirt doesn't seem to correlate to an actual Funko Pop Jack Skellington toy though. The Jack Skellington Funko Pop is #15.
Anyway, I liked the coffin shape around Jack on this shirt and wanted to show you!